1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas and a process for producing the same. More particularly, it relates to a catalyst for purifying an exhaust gas, catalyst which is inhibited from deteriorating the purifying performance even when it is used at elevated temperatures in lean atmospheres, and a process for producing the same.
2. Description of the Related Art
Conventionally, a noble metal, such as platinum (Pt), rhodium (Rh) and palladium (Pd), is used as a catalyst component in a catalyst for purifying an exhaust gas, catalyst which is disposed in an exhaust system of an automobile. These noble metals are loaded on an oxide support, such as alumina (Al2O3), to use. Among them, Pt is more abundant than Rh as a resource, and exhibits a higher catalytic activity than Pd. Accordingly, Pt is utilized mainly in a catalyst for purifying an exhaust gas for an automobile.
However, Pt is oxidized at elevated temperatures in lean atmospheres, is subjected to granular growth, reduces the specific surface area, and consequently its catalytic activity degrades sharply. Recently, the performance of engine has been improved, the chances of high-speed driving have been increased, and further the exhaust-gas regulation has been strengthened. As a result, the temperature of the exhaust gas tends to increase more and more. Hence, it has been desired strongly to develop means for suppressing the granular growth of Pt.
Therefore, the applicant of the present invention proposed in Japanese Unexamined Patent Publication (KOKAI) No. 62-277,150 proposed a catalyst which utilizes a perovskite-type composite oxide which is composed of Pt and a lanthanide element or an alkaline-earth element. In accordance with this catalyst, the thermal deterioration and the alloying of Pt are suppressed with respect to the conventional Pt/ Al2O3 catalyst. Accordingly, the durability of the catalyst is enhanced remarkably.
However, the recent exhaust-gas temperature is considerably high. Even in the catalyst disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 62-277,150, the perovskite-type composite oxide starts degrading thermally in the temperature region exceeding 900xc2x0 C. Hence, in order to cope with the further heightening of the exhaust-gas temperature in the near future, it is necessary to suppress the granular growth of Pt even in the temperature region exceeding 1,000xc2x0 C.
Therefore, the inventors of the present invention made a research and development earnestly, and proposed a catalyst in Japanese Unexamined Patent Publication (KOKAI) No. 10-358. The catalyst is made by mixing a powder, comprising a Pt composite oxide, and a xcex3-Al2O3 powder. The Pt composite oxide contains Pt and one or more element selected from the group consisting of alkaline-earth elements and group 3A elements in the periodic table of the elements. In accordance with the catalyst, since Pt is taken in into the crystals of the composite oxide and it is stabilized therein, it is possible to achieve a high heat resistance of 1,000xc2x0 C. or more.
However, even when the catalyst disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 10-358 is used at elevated temperatures in lean atmospheres, it has been revealed that the Pt composite oxide is subjected to the sintering to a certain extent. Accordingly, it has been found that the specific surface area decreases so that the purifying performance degrades.
The present invention has been developed in view of the aforementioned circumstances. It is therefore an object of the present invention to suppress the sintering of the Pt composite oxide by further enhancing the heat resistance of the Pt composite oxide, and to further upgrade the durability by suppressing the granular growth of Pt clusters.
A first aspect of the present invention carries out the aforementioned objects, and is characterized in that it is a catalyst for purifying an exhaust gas, comprising:
a composite oxide support having a spinel structure expressed by M.Al2O4 in which M is an alkaline-earth metal; and
a platinum structural layer formed on a surface of said composite oxide support and including a matrix composed mainly of an alkaline-earth metal oxide in which platinum clusters are dispersed uniformly.
A second aspect of the present invention is characterized in that it is a catalyst for purifying an exhaust gas, comprising:
a composite oxide support having a spinel structure expressed by M.Al2O4 in which M is an alkaline-earth metal;
a composite oxide layer formed on a surface of said composite oxide support and containing the M and an alkaline-earth metal other than the M at least; and
a platinum structural layer formed on a surface of said composite oxide layer and including a matrix composed mainly of an alkaline-earth metal oxide in which platinum clusters are dispersed uniformly.
A third aspect of the present invention is a process optimum for producing the aforementioned catalysts, and is characterized in that it is a process for producing a catalyst for purifying an exhaust gas, comprising the steps of:
mixing a solution, in which an alkoxide containing an alkaline-earth metal and a platinum compound are solved, with composite oxide support particles having a spinel structure expressed by M.Al2O4 in which M is an alkaline-earth metal, thereby hydrolyzing the alkoxide; and
burning the resulting hydrolyzed products, thereby forming a platinum structural layer on a surface of the composite oxide support particles, the platinum structural layer including a matrix composed mainly of an alkaline-earth metal oxide in which platinum clusters are dispersed uniformly.
Namely, in accordance with the present exhaust-gas purifying catalysts, the coarsening of the Pt clusters can be furthermore suppressed. Accordingly, the present exhaust-gas purifying catalysts can exhibit high purifying performance even after a durability processing in which they are heated at 1,000xc2x0 C., and are extremely good in terms of the heat resistance.
Whilst, in accordance with the present production process, the aforementioned catalysts having good heat resistance can be produced stably and securely.